This invention relates to a high strength bimetallic cylinder and a process for its manufacture. Bimetallic cylinders find particular use in injection molding or extrusion operations.
Injection molding is a process for forming articles from plastics, wherein a heated, fused plastic is forced under pressure into a mold cavity to solidify in the shape and size of the cavity. The charge of plastic material to be injected into the mold cavity must be heated and pressurized prior to injection, and in one approach, the plastic starting material is fed into a hollow cylinder having a screw therethrough. As the screw turns, the plastics forced into a heated zone of the cylinder ahead of a check ring on the head of the screw, so that a predetermined volume of heated, pressurized plastic is prepared for subsequent injection into the mold cavity by a forward movement of the screw and check ring within the cylinder.
Because the economics of injection molding depend upon attainment of long operation lives for the machinery, it is important that the inner lining of the cylinder have a high resistance to wear and corrosion by the heated plastic material. Should the inside of the cylinder wear away so that the inner diameter of the cylinder is enlarged, a clearance develops between the check ring and screw, and the inner wall of the cylinder so that the plastic material leaks back from the pressurized zone, with the result that the necessary pressure for injection molding will not be developed. The cylinder must then be refurbished or replaced, or a larger diameter check ring must be utilized and, in any event, the economic production process is interrupted.
One process currently used for providing a cylinder having a highly wear resistant and corrosion resistant inner lining, while at the same time having an outer portion with high strength and toughness is a nitriding process. Here, the inside of a steel cylinder is nitrided by exposing the inner wall of the cylinder to a nitrogen containing gas such as ammonia, at elevated temperatures. This process results in a relatively thin layer, i.e., 0.005"-0.020", of hardened steel on the inside of the cylinder. Due to the decreasing hardness, with respect to depth, the wear rate will accelerate as material is worn away. For this reason, nitriding is not considered an adequate process for the high wear applications as experienced in extrusion and injection molding equipment.
Another process for providing a cylinder with a lining having improved wear resistance properties uses centrifugal casting. In this process, ingredients suitable for forming an inner layer within the cylinder are loaded with a pre-machined cylindrical outer steel housing. The ends are then sealed, the housing is placed in a furnace at a temperature sufficiently high to melt the inner layer but not the housing, and the cylindrical housing is then rotated rapidly about its axis to distribute the molten ingredients in a continuous layer about the inside of the housing. Upon cooling, the inner layer is metallurgically bonded to the cylindrical outer housing, and the inner layer may then be machined or honed to form a smooth bore of constant diameter to receive the screw and check ring. Boring is preferred to honing since it is less expensive and faster. However, if the lining or inlay is too hard, it cannot be bored and must be ground or honed. Other wear surfacing methods that could be used to apply the liner material to the I.D. of the outer housing include: (1) hot isostatic pressing; (2) plasma transfered arc welding; (3) thermal spray and laser fusion; and (4) direct laser cladding.
The use of ferrous alloys to line a steel housing to form a bimetallic cylinder is known. U.S. Pat. Nos. 2,046,912; 2,046,913; 3,334,996; and 3,658,515 disclose alloys for such use.
The present invention provides a method for making an improved bimetallic, high strength cylinder having a high strength steel backing outer shell with a hard, wear and corrosion-resistant liner or inlay. It should be recognized that the backing steel must have a high yield strength to support high internal pressure without plastic deformation because the hard liner alloy is brittle and will crack with little deformation. It has not previously been possible to manufacture a wear and corrosion resistant liner together with a high strength backing steel. It has previously been necessary to use a heat treated sleeve on the high pressure end of the injection molding cylinder in order to withstand the high stresses at the bore and at the leading edge of the bolt holes that result from injection pressures of 20,000 psi and above.
The use of a heat treated sleeve on the high pressure end of a cylinder used in injection molding has the disadvantage that the remainder of the cylinder is exposed to high pressures that might accidentally occur in processing environments if a cold start were experienced. A cold start occurs when an injection molding machine is shut down with material still in the cylinder. The material cools and becomes a solid plug. When the machine is restarted and, until the plug melts, pressures can be generated in the normally low pressure area of sufficient magnitude to permanently deform the cylinder, rendering it useless.
The reason that it has not been possible to supply a wear and corrosion resistant liner with a high strength backing steel is that when the backing steel is hardened by traditional heat treatment, i.e. quenching from the austenitic phase temperature to room temperature, the hard liner material tends to crack. The traditional practice of hardening steel consists of heating the steel to a high temperature, in the range of 1500.degree.-1900.degree. F. At this temperature, both the liner and backing steel material become austenitic in phase structure. If this material is then rapidly cooled to room temperature, the austenite transforms to another crystalline structure, martensite, which is very brittle and resistant to deformation. However, transformation of austenite to martensite is accompanied by a volume expansion and it is this expansion in the backing steel which would tend to crack conventional hard liner materials such as that described in U.S. Pat. No. 3,658,515 in the bimetallic cylinder during heat treatment.